1. Field of the Invention
The present invention relates to a micro bridge, and more particularly, to a method of manufacturing a conductive micro bridge including a metal-oxide composite.
2. Discussion of the Related Art
In general, methods of manufacturing a micro bridge are largely divided into two groups. One group employs a bulk micromachining technique using anisotropic etching characteristics of silicon. The other group employs a surface micromachining technique using a sacrificial layer.
The micromachining technique uses a semiconductor substrate, and particularly, is applied to a semiconductor substrate on which a matrix of switching field effect transistors are formed. Thus, using this technique, an integrated device can be manufactured.
As an example, a micro bridge is directly formed on a silicon switching matrix using the surface micromachining technique, and a ferroelectric thin film is formed thereon, thereby manufacturing a highly sensitive integrated infrared rays imager.
In a conventional micro bridge formed according to the surface micromachining technique, a sacrificial layer is formed of a phosphosilicate glass (PSG) film. The micro bridge formed thereon is manufactured mainly using a heavily doped polycrystalline silicon.
Thermal conductivity characteristics of thermal flow perpendicular to the surface of the device manufactured by the above technique can be expressed by the following equation: EQU H=kA (.differential.T/.differential.n) (1)
Here, H is heat, k is a conductivity, A is an area of conduction, and (.differential.T/.differential.n) is a thermal gradient in the surface.
The above equation can be simplified, as follows: EQU H=K.DELTA.T (2)
Here, K is a thermal conductivity of a sensing material, and .DELTA.T is a temperature difference, where EQU K=kA/L (3)
Here, L is a length of the thermal conduction portion.
According to the above equations, in order to minimize the thermal loss due to conduction, first, the thermal capacity of the micro bridge must be small. Second, the ratio of A/L must be small, or the path of the thermal conduction must be cut off according to every possible method.
Manufacture of a conventional micro bridge according to the aforementioned surface micromachining technique will be described in connection with FIGS. 1a to 1d.
As shown in FIG. 1a, an insulating film 2 consisting of SiO.sub.2 or Si.sub.3 N.sub.4 /SiO.sub.2 is formed on a semiconductor substrate 1. Then, a PSG film 3a is formed on insulating film 2 through a low pressure chemical vapor deposition (LPCVD) method.
As shown in FIG. 1b, a predetermined portion of PSG film 3a is patterned and etched, thereby forming a sacrificial layer 3. As shown in FIG. 1c, using an LPCVD method, a heavily doped polycrystalline silicon film 4 is formed to a thickness of 1 .mu.m so as to surround sacrificial layer 3. The heavily doped polycrystalline silicon film is used as an electrode or wiring material because of its excellent electric conductivity.
As shown in FIG. 1d, PSG film 3 is removed using fluoric acid, thereby forming an air gap 5 in the removed portion. As a result, the micro bridge consisting of the polycrystalline silicon membrane is formed. The micro bridge is then heat-treated at a temperature of about 1000.degree. C. in order to relieve the interior stress created during the preceding steps, thereby completing the manufacturing process.
The conventional micro bridge formed according to the aforementioned process sequence has numerous deficiencies. For example, the aforementioned conventional micro bridge is formed of a polycrystalline silicon film. When a thin film (e.g., a device such as a thin film infrared rays imager) is manufactured using the polycrystalline silicon film, the crystal growth temperature of a ferroelectric thin film formed on the micro bridge becomes high (about 600.degree. C. and more). Accordingly, when a Pb-group ferroelectric film is used, the Pb component of the ferroelectric film and the Si component of the micro bridge are interdiffused perovskite, or chemically reacted. Thus, the physical characteristics, e.g., the generation of a single perivskite crystal is difficult, thereby deteriorating the dielectric characteristics of the thin film.
Secondly, the manufacturing temperature of the micro bridge and the temperature of the heat treatment for removing the interior stress are high (about 1000.degree. C.). The high temperature deteriorates the FETs of the silicon switching matrix.